The construction of microsensors on microchips is of great interest in many industries because of its potential to reduce size and cost of devices which require the sensing of environmental or other conditions. An accelerometer is just one example of a type of sensor which has wide application possibilities.
Acceleration sensors are known for measuring force or mass or to operate control systems responsive to acceleration. For instance, acceleration sensors may be used in automotive vehicles to deploy air bags responsive to a particular threshold deceleration rate of a vehicle. Acceleration sensors may also be used in the automotive industry as part of active suspension systems in which microcontrollers adjust suspension components responsive to the vertical acceleration of the wheels.
An accelerometer comprises an acceleration-sensing element, or transducer, which is commonly interfaced to resolving circuitry for producing a useful output signal from the output of the transducer. Though the term "accelerometer" is sometimes used to refer to the sensor (or transducer) itself, the term is used herein to denote a complete system including a transducer as well as the resolving circuitry.
Many commercially available accelerometers employ transducers comprising a mechanical or electromechanical element (e.g., piezoelectric, piezoresistive or strain gauge).
Acceleration sensing microstructures embodied on silicon chips have been suggested in the prior art. For instance, U.S. Pat. No. 4,711,128 issued to Boura discloses an acceleration sensor formed by micromachining a fine monocrystal wafer. The sensor comprises a flat mobile mass suspended above the rest of the structure by means of two thin parallel strips situated on each side of the mass. The mass comprises at least one mobile capacitive plate which is disposed between two other capacitive plates which are not part of the suspended microstructure but are fixed on the structure. The mobile plates are charged to a voltage relative to the stationary plates. When the sensor is subjected to an acceleration, the mobile plates move relative to the fixed plates causing a change in capacitance between the mobile plates and each of the fixed plates. The change in capacitance is observed by observing the voltage between the mobile plate and the fixed plates and is a direct indication of the distance of movement of the fixed plate which, in turn, is a measurement of the acceleration.
U.S. Pat. No. 4,705,659 issued to Bernstein et al. teaches a technique of fabricating a free standing thin or thick structure such as an acceleration sensor, including the steps of providing a substrate, forming a layer of carbon on the substrate and depositing a film of polycrystalline material over the layer of carbon. The sandwich structure is heated in an oxidizing ambient to cause the oxidation of the carbon layer, leaving the polysilicon material as a free-standing film.
The prior art, however, does not teach a monolithic accelerometer in which the acceleration sensor as well as the resolving circuitry for producing a useful output are embodied on a single chip or a technique for making such a monolithic accelerometer. Thus, prior art accelerometers require a separate chip or other means containing circuitry for resolving the output of the sensor into a usable signal.